As little as I trust the advice of an aerobics instructor in terms of fighting, I occasionally turn my fist in this manner, particularly when throwing a right and ducking deep to the left...

ie, opponent throws a right, you duck under it moving your entire body to the left while throwing a right on the inside, you twist your hand such that the pinky is ontop and thumb on the bottom with your arm a long way off centre, aiming to the opponents face. I've always found this to be effective and have never had any body mechanics issues throwing a punch this way.

That said, punching like this for a regular jab or cross seems... stupid.

Exactly Triddle, I do the same thing. I've also turned my thumb down at times on the jab if I am throwing it harder and committing more body weight behind it (though it isn't completely corkscrewed either). It's more of a slight turn down from horizontal.